This invention relates to methanol synthesis. Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen, carbon oxides and any inert gasses at an elevated temperature and pressure through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition. Methanol is generally recovered by cooling the product gas stream to below the dew point of the methanol and separating off the product as a liquid. The process is often operated in a loop: thus the remaining unreacted gas stream is usually recycled to the synthesis reactor as part of the synthesis gas via a circulator. Fresh synthesis gas, termed make-up gas, is added to the recycled unreacted gas to form the synthesis gas stream. A purge stream is taken from the circulating gas stream to avoid the build up of inert gasses.
It in order to increase the amount of methanol synthesised, it has been proposed in U.S. Pat. Nos. 5,252,609 and 5,631,302 to subject the make-up gas to a preliminary synthesis step before it is added to the synthesis loop. The throughput may also be increased by operating the loop at a lower circulation ratio, which is defined herein as the ratio of the flow rate of the recycled gas to the make-up gas flow rate. Conventionally circulation ratios are about 3.5:1 or higher, but it is desirable to lower the circulation ratio to less than 2.5, e.g. to about 2, in order to minimise the power required for circulation. However, the use of a preliminary synthesis step, or operation at low circulation ratios, has the problem that the partial pressures of the reactants of the gas fed to the preliminary synthesis step, or to the first synthesis stage of the loop, may be relatively high leading to excessive reaction, and excessive heat evolution in the catalyst bed.
Attempts have been made to increase throughput whilst minimising excessive heat generation in the catalyst bed. For example EP 1080058 and EP 1080059 describe multi-stage processes for methanol synthesis. Synthesis is effected in at least two stages in which the first stage reactor may for example be a gas-cooled type as described in U.S. Pat. No. 4,778,662 fed by recycled unreacted gas to which part of the make-up gas has been added, and the second stage being effected on the mixture of the effluent from the first step and the remainder of the make-up gas in a reactor having indirect heat exchange with pressurised water as a coolant. For operation at low circulation ratios, a limitation of this process is the control of temperature in the first stage gas-cooled reactor. When there is a low circulation ratio in the synthesis loop, there may be insufficient cooling gas going through the tubes of the gas-cooled reactor to ensure adequate removal of the reaction heat and so it is difficult to obtain a satisfactory temperature profile through the catalyst bed. High temperatures are detrimental to catalyst life and lead to the formation of by-products, while excessively low temperatures inhibit the desired reaction.
U.S. Pat. No. 5,827,901 describes a methanol synthesis process wherein preheated synthesis gas is reacted in a first reactor wherein the synthesis catalyst is disposed in tubes cooled by heat exchange with water boiling under an elevated pressure. The resultant reacted gas is then fed to a second synthesis reactor wherein further synthesis occurs. The product synthesis gas is then cooled and methanol is separated and the unreacted gas is recycled and fed, together with make-up gas, as the coolant in the second reactor. In this second reactor the coolant removes heat from the reacting synthesis gas, thus heating the coolant, which is then used as the pre-heated synthesis gas fed to the first heat exchange reactor. In the arrangement of U.S. Pat. No. 5,827,901, the gaseous coolant, i.e. the recycled unreacted gas plus make-up gas, flows in a direction counter-current to the flow of the reacting synthesis gas.